Numerous electromechanical systems have been proposed to control allocation of engine power to automotive vehicle accessories, particularly air conditioners. Generally the intent of such systems has been to cut off the normal diversion of engine power to an accessory device in order to allocate maximum power to the vehicle drive train, especially during intervals of rapid acceleration and hill climbing when vehicle performance requires the greatest possible power transfer to the drive train.
The motivation for developing allocation systems of the prior art stemmed, of course, from either the desire to increase economy or to maximize performance. By temporarily turning off accessory equipment, a portion of an engine's reserve power becomes immediately available to the drive train, and less additional power must be generated to achieve a desired level of vehicle acceleration. Secondly, there has been a recent trend to compromise overall engine capacity in order to meet challenging fuel consumption and emissions standards. Indeed, with the relatively small engines now found in many compact and mid-size cars, undesirable tradeoffs between vehicle economy and reserve power pervade the automotive industry.
Vehicles equipped with relatively small engines normally provide sufficient power for air conditioning, even when the vehicle is fully encumbered with passengers or cargo. Nonetheless, when these small-engine vehicles operate under such laden conditions, they are known to have some difficulty providing sufficient power for rapid climbing of hills or for accelerations of the type necessary to quickly pass other vehicles on the open road. On the other hand, larger and more powerful engines, i.e., those possessing sufficient reserve power under all load conditions, are often less fuel efficient. That is, vehicles equipped with higher output--but less efficient--engines are sometimes referred to as gas guzzlers because they consume substantial amounts of fuel when delivering large and rapid bursts of power.
To compensate for the shortcomings in power and fuel economy exhibited by small and large engines, respectively, several systems have been proposed for disabling automotive accessories from the engine during intervals when peak performance is desired. With such approaches, it has been suggested that an engine with a relatively small power reserve could provide improved driving power under peak demands and a larger engine could be operated in a more fuel efficient range when accelerating.
Generically, these former systems have all required some type of monitoring device to sense vehicle or engine performance, and a control system for disengaging the accessory from the driving engine when a predetermined performance condition is met. By way of example, U.S. Pat. No. 4,305,258 to Spencer suggests using a mercury switch as an "accelerometer" to sense a threshold level of vehicle acceleration. When the vehicle is cruising with the air conditioning system on, the clutch is activated in its normal manner to effect engagement with the engine. Under conditions of sufficient acceleration, it is said that the mercury switch will have its orientation changed by an amount that is sufficient to close an electrical circuit; the circuit will then deactivate the clutch, thereby turning the compressor off. The spencer circuitry can include a timing delay to prevent very brief accelerations from deactivating the clutch. Presumably such a timing delay might also prevent unnecessary deactivation of the clutch as a result of vibrations imparted to the vehicle by rough road conditions.
A feature of systems that respond to vehicle dynamics is that they react to remove the accessory load only after engine output and fuel intake have significantly increased. Initially, while the accessory is still loading the engine down, the engine must work extra hard to accelerate the vehicle. It is only after the vehicle has begun accelerating that full engine power will be allocated to the acceleration. That is, once a threshold level of acceleration has been sensed for a predetermined time period, the engine is relieved of the accessory load. The associated change in engine speed (measured in revolutions per minute and commonly abbreviated as RPM) when the accessory is cut off may cause a jerking-type motion of the vehicle; for those vehicles equipped with automatic transmissions, a jerking change can possibly initiate an undesired and sudden down-shift to a lower gear.
An alternative approach is to monitor when the engine output increases to a level indicative of a significant acceleration. Certain indicators of change in engine dynamics, e.g., increasing RPM or manifold pressure, can be readily sensed to identify conditions characteristic of increasing output. In this regard the system proposed in U.S. Pat. No. 4,369,634 to Ratto for cutting off an air conditioning compressor incorporates a Bourdon tube operatively connected to move in response to changes in engine intake pressure. Such a vacuum-sensitive device can be linked to a mechanical switch in an electrical circuit to momentarily disable the compressor clutch from the engine only during intervals of peak performance.
A further refinement based on these approaches incorporates a pair of monitoring switches, each set to trip at a different threshold value. As suggested in U.S. Pat. No. 4,391,242 to Mashio, such a system may incorporate a pair of pressure switches coupled to timing/decision circuitry. Each switch could be activated at a different pressure value, and the time interval required for the manifold pressure to change between the two values can be used as a criterion for suspending operation of the accessory device. The rate at which the manifold pressure increases from one value to another might be indicative of the rate of change in engine loading and have a positive correlation with the rate of change in vehicle acceleration. Accordingly, operation of the accessory load would be suspended when the time interval between two manifold pressure levels is less than a value which suggests a rapid change in acceleration.
Ideally, any system for disengaging a powered accessory (such as an air conditioner) from a vehicle's engine should be adjustable in order to set desired variables, e.g., economy and comfort, in view of engine and vehicle parameters--and expected operating conditions. At a minimum, these adjustments would be preset by the manufacturer upon installation of the system in a vehicle. However, when the accessory is an air conditioner or a window defogger, frequent readjustment of the system may be often required in order to assure sufficient operation of the accessory under specific vehicle loadings and weather conditions. As a practical matter, any adjustment in the system ought to be under the direct control of the vehicle operator, so that any necessary trade-off between improved driving performance and acceptable accessory operation might be managed.
In the past, systems for automatically cutting off accessory devices have required a mechanical linkage between the chosen monitoring device (e.g., a speedometer, accelerometer or Bourdon tube) and the switching circuitry which controls engagement of the accessory clutch. If this primary linkage were to be housed entirely within the engine compartment, then a remote control means or a secondary linkage reaching from the driver's console to the engine area, would be necessary in order to permit adjustment of the trip point during vehicle operation. In view of these requirements, the commercial success of systems that have the goal of improving vehicle performance by "cutting off" accessory equipment under certain operating conditions has been limited by the cost and inconvenience associated with installation. That is, as an original equipment option, purchase of the system may appear uneconomical to the vehicle owner. As an "aftermarket" purchase, prior art systems have probably seemed even less attractive--due to the expense associated with providing a mechanical linkage and routing it to the driver compartment in the same manner as other linkages that are normally used for dashboard control of vehicle air conditioning and heating systems.